The present invention relates generally superhard materials and more particularly to a superhard compact of nanocrystalline grains of at least one high-pressure phase of B-C-N embedded in a diamond-like amorphous carbon matrix and to a method for preparing the superhard compact.
Superhard materials have a Vickers hardness (Hv), i.e. an indentation hardness, of at least 40 GPa and are widely used as abrasives for drilling, cutting, and other machining applications. Superhard materials often include boron, carbon, nitrogen and oxygen because these light elements have a small atomic radius and form strong and directional covalent bonds that produce tight, three-dimensional networks with extreme resistance to external shear.
Diamond is the hardest superhard material currently known, with an Hv of about 70-100 GPa. However, the actual performance of diamond as an abrasive is somewhat limited. Diamond is an unsuitable abrasive for machining ferrous alloys and has limited applications for high-speed cutting because it is converted into graphite in the presence of oxygen at temperatures over 800xc2x0 C.
Cubic BN (cBN) is another important superhard material. While cBN is widely used for machining fully hardened steels and exhibits much better thermal stability than diamond, it is only about half as hard (Hv=45xcx9c50 GPa) as diamond.
Superhard materials for industrial use are often in the form of sintered polycrystalline composites that incorporate microcrystalline grains of diamond or cubic boron nitride. The grains of this composite are tens to hundreds of micrometers in size, and usually include vacancies, dislocations, and other imperfections that multiply and propagate to form microcracks within individual crystals of a grain, and also along grain boundaries. As the microcracks grow, the materials deform and fracture.
Recently, a new class of materials known as superhard nanocomposites has been reported. Superhard nanocomposites contain superhard nanocrystalline grains embedded in an amorphous matrix. The amorphous matrix provides amorphous grain boundaries that absorb vacancies and dislocations, reduces the surface energy and residual stress among the grains, and permits the relaxation of mismatches between adjacent grains of different phases. While a number of superhard nanocomposites have been reported, no superhard nanocomposite bulk compact having the Vickers hardness of diamond has yet been prepared. Thus, there remains a need for a superhard nanocomposite compact with improved hardness, strength, and performance.
Therefore, an object of the present invention is to provide a bulk superhard nanocomposite compact with a high Vickers hardness.
Another object of the invention is to provide a method for preparing a bulk superhard nanocomposite compact with a high Vickers hardness.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
In accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention includes a superhard nanocomposite compact. The compact consists essentially of nanocrystalline grains of at least one high-pressure phase of B-C-N surrounded by amorphous diamond-like carbon grain boundaries.
The invention also includes a process for preparing a bulk superhard nanocomposite compact consisting essentially of nanocrystalline grains of at least one high-pressure phase of B-C-N surrounded by amorphous, diamond-like grain boundaries. To prepare the compact, a mixture of graphite and hexagonal boron nitride is ball-milled. The ball-milled mixture contains amorphous and/or nanocrystalline graphitic carbon and boron nitride. The ball-milled mixture is encapsulated and sintered at a pressure of about 5-25 GPa and at a temperature of about 1000-2500 K.
The invention is also a bulk, superhard nanocomposite compact prepared by the process of ball-milling a mixture of graphite and hexagonal boron nitride until the mixture is transformed into amorphous and/or nanocrystalline graphitic carbon and boron nitride. The ball milled mixture is encapsulated and sintered at a pressure of about 5-25 GPa and at a temperature of about 1000-2500 K.